Note

This website is presently under development. The primary website for MOPAC is still http://openmopac.net until the majority of its contents have been migrated here.

Z

By default, the number of mers, Z, in a unit cell is worked out from the empirical formula.  Thus, if a unit cell has the empirical formula C₂₄H₂₄, then Z=24.  In those cases where the empirical formula should not be used, e.g., benzene, C₆H₆, the number of mers can be explicitly defined by use of Z=n, here Z=4. Be careful to distinguish between the number of mers in a unit cell and the number in a cluster - Z=n applies to the number in the unit cell.

If Z=n should be used, but is not used, then the heat of formation per formula unit will be in error by a factor, in the case of crystalline benzene, the ΔH_f per formula unit would be 1/6 what it should be.